Mosquitofish may be deadly to several species in Utah waters

Federal and state wildlife officials are urging Utahns not to release mosquitofish (Gambusia affinis) irresponsibly into waters across the state.

These same fish that are being used to help control mosquitoes that may one day spread West Nile virus in Utah may also harm native fish and amphibian populations that are close to being endangered, indicates wildlife officials.

"If used properly, mosquitofish can help control mosquitoes without harming native species," stated Utah Division of Wildlife Resources native aquatic species coordinator, Matthew Andersen. "If they're not, their use could spell trouble for some of Utah's native fish populations."

Problems occur when people release mosquitofish into waters that are occupied by native aquatic species, or that would be good waters to reintroduce native species into, or when mosquitofish escape into these waters on their own.

Andersen says mosquitofish prey aggressively on eggs and young fish and amphibians, including Utah native species the least chub and spotted frog.

These frogs may be especially susceptible to predation by mosquitofish because the amphibians emerge from the egg at a very small size.

Mosquitofish also pose a threat to least chub, which are currently restricted almost exclusively to a few locations in the Snake Valley in west-central Utah and in two spring complexes in Juab County. These fish have eliminated this species from numerous historical habitats, and impeded reintroduction efforts.

Conservation of these species and their habitats is a priority for state and federal natural resource agencies, as both species have been considered for listing under the endangered species act.

"Mosquitofish are almost impossible to eliminate once they're established," advised U.S. Fish and Wildlife Service biologist, Marianne Crawford. "The introduction of mosquitofish into natural systems may permanently change the ecosystem at the expense of native species."

Crawford says that mosquitofish are not a proven control method in all situations. While the fish have been stocked across the United States since the early 1900s, their use is likely to create a false sense of security.

"Introductions of mosquitofish into natural settings such as wetlands and marshes can actually worsen the mosquito problem by eliminating natural mosquito predators that are already there," Crawford indicated.

Crawford says these fish can be effective mosquito controllers in artificial systems that don't contain natural mosquito predators. These include artificial containers such as ornamental ponds and birdbaths, some stormwater management facilities, golf course ponds, some drainage ditches, excavated sites such as farm ponds, sewage lagoons, and wastewater facilities.

"There are insecticides that are target-specific to mosquito larvae, and pesticides with mechanical actions that suffocate larvae by covering the water surface for up to 10 days. Then they bio-degrade," Crawford explained. "Even though there may be minimal short-term effects on native aquatic species, selection of environmentally-compatible insecticides and pesticides can be a lot safer in the long run."

Eliminating artificial sources of standing water, where possible, is still the best way to control mosquitoes.

"Mosquitoes cannot be completely eliminated by any single control method, or combination of methods, so it's important to wear protective clothing and use insect repellent as needed," Crawford concluded.